Understanding of language and communication is crucial to understanding mental retardation because they control, express, or mediate much of our three major developmental spheres: the intellectual, social, and physical. For this reason, language and communication must be a central focus of research in mental retardation. The proposed research was guided by a conceptualization of human development that stressed the centrality of the developmental cusp an intersection in the interaction of behavior, skill, perception, or motivation, that is crucial to what should come next in successful negotiation, however, permits a significant set of subsequent developments to occur. The cusps chosen to be studied in the proposed research appear critical to the process of communicative development and are representative of a diversity of points (from early and rudimentary to late and sophisticated). They are representative of the problems of a diversity of people including those with mental retardation. They span the intellectual, social, and physical spheres of development, and seem amenable to realistic intervention. Thus, the results of our research can contribute directly to preventing and/or ameliorating significant aspects of mental retardation. Our plan is to enable persons with mental retardation to: learn how to demonstrate their preferences and choose among them, primarily by learning to discriminate reinforcement contingencies (Project I, Saunders & Saunders); put themselves more frequently in contexts that support and enhance their attempts to communicate, primarily by developing positive social relationships (Project II, Sherman & Sheldon); learn to maintain communication attempts until communication is achieved primarily by learning to make communication repairs (Project III, Brady); and, use language-life skills to affect their own problem-solving abilities, primarily by learning to self-instruct (Project IV, Baer & Grote). Hart (Project V), working from an extensive database of language samples collected in prior years, proposes to continue to increase our understanding of the similarities and differences between the language development of children with Down syndrome and typically developing children, primarily by examining the development of declarative sentences as they may be related to the enhancement of expressive language.